Since the frequency band resource of the transmission channel is always restricted in a communications system, it is one of the most important indices for the communications system to seek to enhance transmission efficiency. To guarantee anti-interference performance and spectrum utilization ratio of the communications system, channel coding and symbol modulating technologies are widely applied in the communications system.
Quadrature amplitude modulation (QAM) is a modulation mode frequently employed in the communications system. QAM is a technology of modulation combining amplitude with phase, and makes simultaneous use of the amplitude and phase of a carrier to transmit information bits, so that higher frequency band utilization ratio can be achieved in the condition of identical minimum distances between constellation points. Similar to other digital modulation modes, a QAM modulation symbol set can be conveniently expressed by a constellation, on which each constellation point corresponds to one point in the symbol set.
On the other hand, for instance with regard to a relay system, a relay link is established between a mobile or stationary relay station (RS) and a stationary base station or a stationary RS in the network. A mobile terminal (also referred to as the mobile station) makes use of the relay link of the mobile or stationary relay station to which it belongs to communicate with the base station. Since the RS is capable of communicating between network devices to interchange information of a plurality of mobile terminals and to collect and distribute communication data between the terminals and the base station, it is possible to achieve maximization of the communication link efficiency between the network devices and to provide better communication quality.
The relay station processes signals mainly in two modes. The simplest one is the “Amplify and Forward” (AF) mode, whereby the relay station merely amplifies the received signal according to a certain coefficient. Another mode is referred to as “Decoder and Forward” (DF), whereby the relay station demodulates the signal and decodes it as the original information, and then encodes and modulates the same again for transmission. This mode can enhance signal quality in the case the channel condition is good, but if the relay station erroneously decodes, error propagation will be present in the forwarded signal, and it is also impossible for the mobile station to recover. There is also “Estimate and Forward” mode among others.
In another technology called cooperative diversity, both receiving and forwarding of data are relayed and enhanced via the mobile terminal. There are also the aforementioned two modes of AF and DF for processing the signal.
In the foregoing mechanism for receiving and forwarding data, the data receiving operation and forwarding operation can be performed either separately or in hybrid. Three relatively typical forwarding modes are respectively shown in FIGS. 1-3.
FIG. 1 illustrates the conventional bidirectional relay process. As shown in FIG. 1, the base station and the mobile station respectively transmit data to the relay station at different timings (frequencies), and the relay station then forwards data respectively to the base station and the mobile station at different timings (frequencies), in which altogether four periods of time are needed. Specifically, in the example as shown in FIG. 1, the base station transmits to the relay station data to be forwarded to the mobile station in the first time slot, and the relay station receives the data. The mobile station transmits to the relay station data to be forwarded to the base station in the second time slot, and the relay station receives the data. In the third time slot the relay station forwards the data received from the base station at the first time slot to the mobile station, and in the fourth time slot the relay station forwards the data received from the mobile station at the second time slot to the base station.
FIG. 2 illustrates the mode of separated reception and combined transmission. As shown in FIG. 2, the base station and the mobile station respectively transmit data to the relay station at different timings (frequencies), and the relay station receives the two branches of data and then combines them into one branch of data to be simultaneously (or in the same frequency) forwarded to the base station and the mobile station. Upon reception of the data forwarded by the relay station, the base station and the mobile station decode or recover the data. Altogether three periods of time are needed. Specifically, in the example as shown in FIG. 2, the base station transmits to the relay station data to be forwarded to the mobile station in the first time slot, and the relay station receives the data. The mobile station transmits to the relay station data to be forwarded to the base station in the second time slot, and the relay station receives the data. In the third time slot the relay station combines the data received from the base station at the first time slot with the data received from the mobile station at the second time slot, and then forwards the data to the base station and the mobile station simultaneously (or in the same frequency).
FIG. 3 illustrates the bidirectional hybrid forwarding mode, whereby the base station and the mobile station transmit data to the relay station at the same timing (frequency), and the relay station forwards the relay data to both of the base station and the mobile station simultaneously (or in the same frequency) after receiving and processing the data, in which altogether two time periods are needed. Specifically, in the example as shown in FIG. 3, the base station transmits to the relay station data to be forwarded to the mobile station in the first time slot, and at the same time the mobile station transmits to the relay station data to be forwarded to the base station. The relay station simultaneously receives the data from the base station and the data from the mobile station. In the second time slot the relay station combines the data received from the base station with the data received from the mobile station at the first time slot, and then forwards the data to the base station and the mobile station simultaneously (or in the same frequency).
As can be seen from FIGS. 1-3, the bidirectional hybrid forwarding mode occupies the least channel resource (time period), and is relatively high in spectrum utilization ratio, but data processing thereof is relatively complicated.
In the prior art DF mode, hybrid processing of the relay station subjects the decoded data from the base station and the mobile station to a bit XOR operation to organize into a combination signal to be subsequently encoded for transmission. This requires that the coding modulation modes of the two links from the mobile station to the relay station and from the base station to the relay station be the same. When signal-to-noise ratios of the two links differ not much, this solution is feasible. But due to time variation of the mobile channel, when signal-to-noise ratios of the two links differ relatively greatly, this method will cause certain loss of throughput.
FIG. 4 is a block diagram schematically illustrating a prior art relay system that performs Decoder and Forward (DF). As shown in FIG. 4, the relay system that performs DF mainly includes a base station transmitter 100-1, a mobile station transmitter 101-1, a base station-relay station channel 102, a mobile station-relay station channel 103, a relay station 104, a base station receiver 100-2 and a mobile station receiver 101-2. Data transmitted from the base station transmitter 100-1 and the mobile station transmitter 101-1 reaches the relay station 104 via the channel 101 and the channel 102, with noise being mixed therein en route. As shown in FIG. 4, data from the base station and mixed with noise is represented by A, while data from the mobile station and mixed with noise is represented by B. After the relay station 104 receives the signal A from the base station and the signal B from the mobile station, it combines the received signals into a transmission signal (C) of the relay station, and subsequently respectively forwards the signal to the base station and the mobile station. The base station and the mobile station respectively detect and receive the combination signal from the relay station.
FIG. 5 is a block diagram schematically illustrating data processing at the relay station 104. As shown in FIG. 5, signals from the base station and the mobile station are firstly respectively received (including demodulating and decoding, for instance). The receiving mode of the relay station is not defined in this paper, as it can be either independent receiving (namely respectively receiving the information from the base station and the mobile station at different timings/frequencies, as the mode shown in FIG. 2) or receiving at the same time/in the same frequency (as the mode shown in FIG. 3).
The relay station performs the combination operation after receiving the data. One typical data combination mode is bit XOR, for instance, if the two branches of data received by the relay station are sequenced respectively as {a0,a1,a2,a3, . . . } and as {b0,b1,b2,b3, . . . }, the operation of the combining device 1040 in FIG. 5 will beci=ai⊕bi i=0, 1, 2, . . .  (1)where ⊕ is the symbol of the XOR operation. The combined bits are transmitted after having been recoded and modulated.
At the receiving ends of the base station and the mobile station, since one branch of the combination signals forwarded by the relay station is the local original data (namely locally transmitted signal, for instance as regards the base station, it is the signal A free from noise, and as regards the mobile station, it is the signal B free from noise), another branch of the signal (namely the signal desired to be received) can be recovered by performing XOR operation on the received data and the local original data (hereinafter referred to also as the local reference signal), so as to achieve data forwarding. FIG. 6 illustrates the detection and reception processes of the base station and the mobile station. In comparison with the general receiver system as shown in FIG. 1, a local separating device 201 is added downstream of the decoding unit. The separation operation that corresponds to the XOR combination operation in FIG. 5 is as follows:âi=ĉi⊕bi i=0, 1, 2, . . .  (2)where ĉi is the bit of the received combination information, âi is the bit of the information desired to be received after separation (also referred to as desired information bit), and bi is the local information bit (reference information bit) and provided by a local signal storing unit 202.
The conventional XOR operation demands that the length of the two bit series be the same, and this requires that their modulation and coding scheme also be the same. However, when signal-to-noise ratios of two links differ relatively greatly, it is possible to determine the modulation and coding scheme only in accordance with the link having inferior signal-to-noise ratio, and this reduces throughput of the system.
Reference documents of the present invention are listed below. These documents are herein incorporated by reference, as if they were described in detail in the Description of the present invention.    1. [Patent Document 1]: Xue Feng, et al., Combining packets in physical layer for two-way relaying (US 080219251 A1)    2. [Patent Document 2]: Liu; Zhixin, et al., Compress-forward Coding with N-PSK Modulation for the Half-duplex Gaussian Relay Channel (US 070217541 A1)    3. [Non-Patent Document 1]: P. Larsson, N. Johansson, K. E. Sunell, “Coded bi-directional relaying”, the 5th Scandinavian WS on Wireless Ad-Hoc Networks (AdHoc '05), Stockholm, Sweden, May 2005.    4. [Non-Patent Document 2]: Petar Popovski, et al., “Wireless Network Coding by Amplify-and-Forward for Bi-Directional Traffic Flows,” IEEE Communication Letters, Vol. 11, NO. 1, January 2007.    5. [Non-Patent Document 3]: Sachin Katti, et al, “XORs in The Air: Practical Wireless Network Coding,” SIGCOMM '06, Sep. 11-15, 2006, Pisa, Italy.